The Stalker: An Easy DIY Double Tracker

[Ripthorn]

I've been on a bit of a delay kick (see EchoWreck and T60) and thought "why not try a double tracker?" However, one reason there are so few of them out there is that the PT2399 doesn't do short enough delay times for a lot of double tracking. It's got a 29 ms or so minimum time. I was repairing a CE-5(t) that uses the ES56028 and found it had very low delay times, but it's a 40 pin chip. Turns out there is a 16 pin ES56033. I bought five on Aliexpress to try out and the Stalker came out of it.

The ES56033 has a minimum delay time of 14 ms, which works quite well for a minimum delay time in a double tracker. I designed the Stalker with a ~55ms max delay time so that it just gets into slap back territory. It has controls for time (distance), mix (stealth), and modulation (creepy) as well as a switch that disables the modulation LFO. The modulation is very mild, as the typical "pin 2 hack" or other modulation schemes used on a PT2399 don't work on the ES56033 due to how delay time is set.

Apart from pin assignments, the ES and PT are fairly similar in architecture. For a more detailed explanation on how it works and design choices, I have a full on article at the webpage for this project at https://scientificguitarist.wixsite.com/home/the-stalker. It also has build documentation, a perf board layout, and suggestions on how to do things like stereo output, etc. Here is a quick and dirty demo video:

And the schematic (may be hard to read, I've got a PDF available at the webpage).

[ElectricDruid]

Nice!

The modulation is interesting. At first glance it looks like the "VCO" is probably a simple Schmitt inverter oscillator. Assume it's output is high for a moment. In that case feeding extra current to the cap should get the frequency to increase. The problem then is that when the inverter output flips low, the extra current is now acting *against* the previous modulation, since it is trying to charge the cap while the inverter tries to discharge it.

Presumably the modulation only works at all because there's some mismatch in the switching levels and therefore some asymmetry in the up/down sections of the clock waveform, like shown here:



Would that work? Does that make sense? I feel like it does, but I can't quite grasp it currently.

[Ripthorn]

It seems like you would be right, but I can't be certain. I do know that the modulation has a small effect, but it's small. It does just enough to not make it sound like a super clean, straight up delay, but I can't really describe what the effect is like. That cap to ground is what I assume is the issue with the modulation, and that cap MUST be there, believe me, I tried to get away without it

[anotherjim]

Sounds cool.
I'd agree with Tom that's the most likely operation of the LFO on the clock rate. However you look at it, if it alters the charge on the fixed cap or the DC offset to the Schmitt trigger, it's the same thing. If you wanted a lot of mod depth, I think a Vactrol in the timing resistor is the easy thing to do - but this an effect best served subtle.

[ElectricDruid]

There's a couple of other clues in the datasheet for the ES56033:

https://datasheetspdf.com/pdf-file/855216/Itri/ES56033/1

It says that the internal clock rate can be measured on pin 3. So on the diagram I posted, that'd be pin 2 on the left (junction of the R and C) and that's pin 3 on the right (Schmitt inverter output).

It'd be *possible* to use the pin3 signal to drive a stage that flipped the sign of the LFO when required to ensure that the modulation was always in the right direction w.r.t. the clock, but it's a bit of a faff - certainly not as easy as just ramming some extra volts into some pin or other, and also not as easy as sticking a vactrol across the R.

If you could disable the internal inverter, you could feed a clock into pin 3 instead, but *unless* you can disable that inverter, you're going to be fighting its output stage.

Another thing I notice from the datasheet is the clock rate is given as 230KHz to 400KHz - not even a full octave's range. So *any* modulation on this chip, even if using the full range of the thing, is not going to be extreme when compared to things like the MN3207 that can cover five octaves or more.

[John Lyons]

How is the regeneration/repeats set in this circuit?

[Ripthorn]

John, I don't have any repeats in this circuit. I'm trying to stay true to the double tracking. Implementing it with this chip is very similar to a pt2399 circuit.